This invention is directed to measuring instruments and, more particularly, to optical measuring gauges.
The present invention was developed for use in monitoring the change in the contour of the surface of a part (e.g., elevational changes) while the part is being machined by a lathe or other machine tool, for example. Since the invention was developed for use in machine tool environments, it will be described and illustrated in such an environment. However, it is to be understood, and will be readily recognized by those skilled in the art and others, that the invention is also useful in other environments. In general, the invention is useful in any environment where it is desirable to detect small changes in the contour of a moving surface along an axis normal to the surface, without contacting the surface. For example, the invention can be used to detect the transverse movement (vibration) of the rotating shaft of a ship in order to determine when said transverse movement exceeds an acceptable level. The present invention can also be used to provide an automatic focusing device useful in the precise photography of a moving surface, for another example.
In many environments, it is desirable to precisely measure the change in surface contour of a moving item or part without contacting the surface whose change is to be measured. Such a requirement is particularly important in environments where unwanted electrical grounds may occur if the sensing device contacts the moving surface and in environments where a contacting sensor may score or mar the moving surface. Such a requirement is also important in environments where the part is rough and moving rapidly, because contacting sensing elements will rapidly wear away and be destroyed. This latter problem is of particular significance in machine tool environments.
In the past, small displacements in the surface contour of a rotating cylinder or a sliding, flat surface have been measured using capacitive or inductive transducers as the sensing element. These systems have the disadvantage that they are effective only over short distances. Thus, it is impossible to use them when the displacement range of the moving surface may vary over several inches, or when it is impossible to position such devices near the surface because of other objects or items. Moreover, in some environments inductive and capacitive devices cannot be used because it is impossible to shield them from the effects of extraneous magnetic and electrostatic fields located in the measuring vicinity, which act to distort the resultant information.
In environments wherein a noncontacting sensor is required, and capacitive or inductive transducer sensors cannot be used, the prior art has proposed the use of optical gauges, many of which use interferometer principles. Optical gauges using interferometer principles have the disadvantage that they measure contour changes only, and not absolute values of surface contour. Such a measuring technique also has the disadvantage that any temporary interruption of the light transmission path during measuring will render the resultant measurement in error and, thus, useless. In addition, interferometric techniques require specular (highly polished) surfaces on parts to be measured, along with precise alignment prior to measurement.
Other prior art optical gauges proposed for use in machine tool environments have mounted a retroreflector on the tool holder. Light is directed toward the retroreflector and reflected back to a sensor. The transmission time is measured and used to determine the position of the retroreflector with respect to the light source and/or the photodetector. However, this arrangement also has disadvantages. Specifically, prior art optical gauges using retroreflectors measure tool-holder position, not part position. While the information developed is related to changes in the surface of the moving part normal to the beam, the information is subject to error because it does not compensate for tool wear, tool holder wear, or any misalignment between the position of the retroreflector and the actual part. More specifically, retroreflector type optical gauges do not compensate for any mechanical errors present between the retroreflector and the part being machined, said errors being primarily caused by wear and misalignment.
Therefore, it is an object of this invention to provide a new and improved optical measuring gauge.
It is another object of this invention to provide a new and improved noncontacting surface contour measuring gauge.
It is a further object of this invention to provide a new and improved optical gauge for measuring changes in the surface contour of a moving part.
It is still another object of this invention to provide an optical gauge that measures the absolute value of surface contour changes.
It is a still further object of this invention to provide an optical surface contour measurement gauge that makes measurements directly from the surface of a moving part.
It is a still further object of this invention to provide an optical gauge suitable for use in a machine tool environment for measuring changes in the contour of the surface of a moving part, as the part moves past a point.